The production, absorption, and excretion of intestinal gas are being investigated to obtain information concerning the function and malfunction of the gastrointestinal tract. The absorption of 133 Xenon from the rectum is used to "mark" portal blood and thus determine if blood in the stomach is derived from bleeding varices. H2 excretion on the breath is being used to study carbohydrate malabsorption and intestinal transit time. Mucosal blood flow to the gut is being studied by means of gas absorption rates.